Zero IF receivers for radio signals are well known in the art, and are amply described in publications such as Proc. IRE 44 (1956), pages 1703-1705, and U.S. Pat. No. 2,928,055, both of which are incorporated herein by reference. In a zero IF radio receiver, a received FM or AM signal is mixed with the output of a down conversion oscillator to translate the received signal to baseband. Equal positive and negative frequency excursions about the carrier frequency result in the same deviation frequency; however, the polarity of the modulation can no longer be determined without some phase reference signal. To provide that reference signal, two substantially identical signal paths known as I+Q paths are provided in which the received signal is down converted to baseband, low pass filtered to remove the sum products of mixing as well as undesired adjacent channel signals, and up converted to an output frequency.
The down and up conversion oscillators for one path are in phase quadrature with their counterparts in the other path. When the outputs of the two paths are then summed the side bands cancel in such a manner that the modulation polarity of the original received signal is retained, though translated to a new, predetermined output frequency. In effect, the received signal is translated from an incoming frequency to baseband, filtered to remove interfering adjacent signals, and reconverted to an output frequency in which conventional FM demodulation can take place.
Receivers known as superheterodyne receivers use a different technique. A superheterodyne receiver converts an incoming radio signal to one or more intermediate frequencies in which amplification and frequency selection are more easily performed than at the frequency of the received signal. Typically, the carrier frequency of the signal is converted once or twice in successive stages for eventual demodulation. The intermediate frequency, typically a high frequency determined by the higher order spurious responses of the mixer, is selected at each stage through a band pass filter.
It is difficult to miniaturize superheterodyne receivers because high-Q crystal or ceramic band pass filters cannot be easily integrated in monolithic form. This is especially true for high frequency filters. Zero IF receivers, on the other hand, can be miniaturized because frequency selectivity is achieved through low pass filtering. Low pass filters are readily fabricated in monolithic form, especially when cutoff is at a low frequency, typically on the order of the bandwidth of the signal. Thus, zero IF receivers have the advantage of smaller size over conventional superheterodyne receivers.
Although they do have some advantages, zero IF receivers have a number of drawbacks that have heretofore limited their use. Receivers of this type typically include several high gain amplifiers typically subject to automatic gain control (AGC). This controlled amplification preserves linearity in the zero IF amplifiers so that the modulated signal is accurately recovered when the two signals are recombined. The problem is that DC offsets inherent in the amplifiers and other elements of the receiver circuit can cause the amplifiers to saturate. One recognized method for overcoming this problem is to AC couple the receiver elements to block their DC offsets. The AC coupling creates a DC notch around zero frequency that dampens the lower frequencies. The portion of the modulated signal centered around the carrier frequency is thus lost when the modulated signal is translated to the second IF frequency for detection.
For signal modulation formats such as FM and single sideband (SSB) the DC notch causes distortion since the notch frequencies contain signal information. Moreover, with automatic gain control, the rest of the signal is overamplified to compensate for the lost signal within the notch. This additional amplification also distorts the received signal. For these and other reasons, direct conversion receivers are primarily used for processing digital information such as with frequency shift keying (FSK) where loss of the modulated signal portion about zero frequency is not critical.
Several variations on the described zero IF receiver design have been tried for overcoming these drawbacks to analog use. For example, U.S. Pat. No. 4,653,117 teaches placement of amplification and limiting functions outside the dual signal paths at a non-zero intermediate frequency. Gain within the signal paths is thus intentionally avoided to prevent the need for AC coupling and the resultant partial loss and distortion of the demodulated signal. DC offsets are also minimized by the use of differential circuits in the zero IF stage. The drawback to this approach, however, is the need for additional circuitry to provide the needed gain. The use of differential amplifiers and the addition of amplifiers and limiters beyond the summing junction of the signal paths makes the receiver circuit somewhat larger and more complex to design and fabricate.
Copending patent application Ser. No. 07/229,976, now U.S. Pat. No. 4,944,025 entitled "DIRECT CONVERSION FM RADIO RECEIVER WITH OFFSET" filed Aug. 9, 1988 describes the use of an offset frequency to eliminate the notch problem. However, the use of an offset frequency imposes added requirements on the filters in the system and thus has associated disadvantages from a practical point of view.
Since an advantage of zero IF radio receivers is that they can be constructed in monolithic form, an area of practical application for such receivers is in devices which require a minimum of size and space. A paging system which uses an FM radio receiver contained in a wrist watch is shown in Gaskill, et al., U.S. Pat. No. 4,713,808. The FM receiver circuitry disclosed in the Gaskill patent operates intermittently and at a very low duty cycle. The receiver is active for about 33 milliseconds and is then inactive for about two minutes.
As stated above, it has been necessary to provide AC couplings for automatic gain control amplifiers in each signal path of a zero IF receiver due to errors created by DC offsets in the amplifiers which can lead to saturation. For some time it has been known that amplifiers may be designed which automatically compensate for such errors which are caused by drift. Such amplifiers are generally known as auto zeroing amplifiers and an example of such an amplifier is described in the May 11, 1989 issue of Electronic Design magazine. In amplifiers of this type a zero adjustment is made during an adjustment cycle which may be then followed by an operating cycle.